JP2691124B2 - Optical gas analyzer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical gas analyzer for measuring the concentration of a gas to be measured by detecting the light absorption intensity using laser light, and more particularly to the optical frequency of laser light. The present invention relates to an optical gas analyzer that prevents deterioration of measurement accuracy due to optical interference noise of laser light by averaging light absorption intensities for a plurality of specific frequencies obtained by sweeping.

[0002]

2. Description of the Related Art Conventionally, as a gas analyzer for measuring a gas concentration, several devices such as a gas chromatography, a mass spectrometer and an infrared spectroscopic analyzer are known. Of these gas analyzers, at the site of gas concentration measurement and analysis, the structure is not large-scale, simple and relatively inexpensive, easy to handle and maintain, and suitable for real-time measurement. Infrared spectroscopy analyzers have been widely used.

Here, the case of measuring the gas concentration will be described using an example of the light absorption spectrum shown in FIGS. 2A and 2B. 2A shows a light absorption spectrum for the sample gas a, and FIG. 2B shows a light absorption spectrum for another sample gas b. The horizontal axis shows the optical frequency and the vertical axis shows the light absorption intensity. 2A and 2B exemplify a case where the light absorption spectrum lines of the sample gases a and b are very close to each other on the optical frequency axis.

For a single type of sample gas, if the optical absorption spectra shown by the solid lines a and b in FIGS. 2A and 2B are generated due to the vibrational and rotational energy transitions of the molecules, respectively, a mixture of a plurality of gases is obtained. For a sample gas composed of a body, for example, if the sample gas is a mixture of the sample gases a and b, a light absorption spectrum in which solid lines a and b of FIGS. 2A and 2B are superposed occurs. It will be.

In detecting the optical absorption spectrum, if the optical frequency resolution of the infrared spectroscopic analyzer is sufficiently high,
2A and 2B, it is possible to detect the light absorption spectra as shown by the solid lines a and b, and it is possible to detect each spectral line separately, but when the optical frequency resolution is low, 2A, the detection results shown by the broken lines C and D in FIG. 2B are obtained, and it becomes impossible to detect individual light absorption spectrum lines for a single type of sample gas.
Therefore, when the light absorption spectrum lines are extremely close to each other on the optical frequency axis, it becomes difficult to detect and separate the light absorption intensity for each gas. In particular, this effect becomes remarkable when measuring a gas having a low abundance ratio in the sample gas.

An infrared spectroscopic analyzer, which is a typical example of a conventional optical gas analyzer, has a low optical frequency resolution of a diffraction grating used as an optical frequency discriminator, and when a mixed gas is used as a sample gas, In particular, the performance of separating and detecting the superposed light absorption spectral lines was not sufficient.

On the other hand, there is a laser spectroscopic analyzer as an optical gas analyzer having the same operability as the infrared spectroscopic analyzer. As shown in FIG. 3, this laser spectroscopic analyzer comprises a laser light source 2, a detector 4, a control circuit 6, a processing circuit 8 and a conversion circuit 10, and a sample gas is provided between the laser light source 2 and the detector 4. Place the sample cell 12 into which
The laser light 14 emitted from the laser light source 2 is supplied to the sample cell 1
It is configured such that the detector 4 detects the transmitted light 15 that irradiates 2 and is transmitted through the sample cell 12.

In FIG. 3, a laser light source 2 emits a laser beam 14 having a very narrow optical spectrum width, and the optical frequency of the laser beam 14 is controlled by the control circuit 6 to be a specific optical frequency of the sample gas. Swept within range. The laser light 14 is applied to the sample cell 12, absorbed by the sample gas, and then transmitted through the sample cell 12 as transmitted light 15, and the light intensity is detected by the detector 4.

Here, the processing circuit 8 receives the optical frequency information 16 regarding the laser beam 14 output from the control circuit 6 and the optical intensity signal 18 output from the detector 4 for the laser beam 14 of this optical frequency. From this, the light absorption intensity of the gas contained in the sample gas at a specific optical frequency is obtained, and this is obtained.
Convert to concentration data at.

In the laser spectroscopic analyzer of FIG. 3, since the light source for emitting the laser light 14 having an extremely narrow optical spectrum width is used as the laser light source 2, the optical frequency resolution is sufficiently high and is shown in FIG. As shown in the example, even if there are multiple gases in the sample gas that have very close absorption lines on the optical frequency axis, each absorption line can be detected separately. is there.

[0011]

However, since the laser beam used in the conventional laser spectroscopic analyzer shown in FIG. 3 has an extremely narrow optical spectrum width, it has a high coherence and an optical resonance on the optical path. The presence of the lamp has the disadvantage that the light intensity varies due to interference.

That is, in the laser spectroscopic analysis device of FIG. 3, it is impossible to expect ideal transmissivity for the entrance / exit surfaces of the optical elements such as the sample cell 12 and the detector 4, and in reality it is very small. As a result of reflection, the optical element eventually constitutes an optical resonator. On the other hand, the optical frequency of the laser light 14 emitted from the laser light source 2 is selected according to the specific optical frequency of the sample gas. Therefore, the intensity of light incident on the detector 4 includes not only absorption by the sample gas but also optical interference noise that fluctuates with the variation Δf (= c / 2nL) of the optical frequency f as a cycle. Note that c is the speed of light, n is the refractive index, and L is the optical resonator length.

In measuring the concentration of the sample gas, it is required to detect a minute change in the light intensity due to light absorption with high accuracy. However, as described above, the periodic fluctuation of the light intensity causes an optical interference noise. Since it is superimposed on the detection signal of the light absorption spectrum, it has been an obstacle to improving the measurement accuracy. Moreover, the decrease in measurement accuracy caused by the optical interference noise is extremely large as compared with other optical noises and electrical noises, and has been a major factor in determining the gas concentration detection sensitivity.

Therefore, in the present invention, the above problems are solved, and even if a plurality of gases having optical absorption spectral lines close to each other on the optical frequency axis are present in the gas to be measured, they have high resolution and mutually absorb the optical absorption. An object of the present invention is to provide an optical gas analyzer which can easily realize highly accurate measurement of each gas concentration without being affected by the spectrum and reducing optical interference noise of laser light.

[0015]

In order to achieve the above object, the present invention provides a laser beam having an optical spectrum width sufficiently narrow with respect to the optical absorption spectrum line width of the gas to be measured and the optical absorption spectrum line interval. Generating a laser light source, a selection circuit for selecting optical frequency information of a plurality of optical absorption spectral lines of the gas to be measured, and controlling the laser light source, sweep the optical frequency of the laser light in the range of the optical frequency information A control circuit, a detector for detecting the light intensity of the laser light transmitted through the gas to be measured, a light for the optical frequency of a plurality of light absorption spectral lines of the gas to be measured from the light intensity detected by the detector It is characterized by comprising a processing circuit for extracting the absorption intensity and an averaging circuit for averaging the light absorption intensities with respect to the plurality of light absorption spectral lines extracted by the processing circuit.

[0016]

In the optical gas analyzer of the present invention, the optical frequency information of a plurality of optical absorption spectral lines of the gas to be measured is selected, laser light is irradiated to the gas to be measured based on the information, and the transmitted light is transmitted. To detect. Next, the detected plural light intensity data are averaged to smooth the optical interference noise of the laser light, and the density is obtained from this.

[0017]

EXAMPLES Regarding an optical gas analyzer according to the present invention,
Embodiments will be described in detail below with reference to the accompanying drawings. FIG. 1 is a diagram showing the configuration of an embodiment of the optical gas analyzer according to the present invention. In FIG. 1, reference numeral 2
Is a laser light source, 4 is a detector, 6 is a control circuit, 8 is a processing circuit, 10 is a conversion circuit, 12 is a sample cell into which a sample gas is introduced, 28 is a selection circuit, and 30 is an averaging circuit.

The laser light source 2 generates a laser beam 14 having a light spectrum width sufficiently narrow with respect to the light absorption spectrum line width of the sample gas and the interval between the light absorption spectrum lines. The detector 4 uses the laser light 14 transmitted through the sample cell 12.
Detect the light intensity of.

Optical frequency information of a plurality of optical absorption spectrum lines is registered in advance in the selection circuit 28 for each gas to be measured, and optical frequency information of all optical absorption spectrum lines of the sample gas is registered. 16 is selected and output to the control circuit 6, and the sweep range of the optical frequency is set. Further, the selection circuit 28 outputs the optical frequency information 37 relating to the specific sample gas to the averaging circuit 30. Control circuit 6
Sweeps the optical frequency of the laser light source 2 within the sweep range and supplies the optical frequency information 16 being output to the processing circuit 8. The processing circuit 8 extracts the light absorption intensity corresponding to the optical frequency information 16 from the light intensity detected by the detector 4. The averaging circuit 30 performs an averaging process of the light absorption intensity of the specific sample gas supplied from the processing circuit 8. The conversion circuit 10 converts the averaged light absorption intensities into density data.

Next, the operation of the optical gas analyzer of this embodiment will be described.

First, when the sample gas to be introduced into the sample cell 12 is specified, the optical frequency information 16 relating to the sample gas is specified.
Is selected by the selection circuit 28 and transferred to the control circuit 6. The control circuit 6 controls the laser light source 2 within the range of the optical frequency information 16 and outputs the laser light 14.

The laser light 14 emitted from the laser light source 2 and applied to the sample cell 12 into which the sample gas is introduced is absorbed by the sample gas inside the sample cell 12 and then transmitted light 1
It enters the detector 4 as 5. The detector 4 detects the light intensity of the transmitted light 15 and outputs a light intensity signal 18 to the processing circuit 8. In the processing circuit 8, the optical frequency information 1 from the control circuit 6
6 and the light intensity signal 18 output from the detector 4, the light absorption intensity corresponding to each light absorption spectrum line is obtained at a plurality of specific optical frequencies.

The averaging circuit 30 further extracts a light absorption signal corresponding to the light frequency information 37 for the specific sample gas selected by the selection circuit 28 from the light absorption intensity obtained by the processing circuit 8 and extracts these signals. Average out. Then, the averaged light absorption intensity is converted into density data in the conversion circuit 10. In this case, there is generally no correlation between the spacing of the spectral lines and the optical interference noise period of the laser light,
Since the light absorption intensities including the optical interference noise are averaged, the above-described configuration enables highly accurate measurement of the gas concentration with reduced optical interference noise. The selection circuit 28 and the averaging circuit 30 can be easily configured by using well-known elements that have been widely used in the past, such as a microcomputer and an analog operation circuit.

[0024]

According to the optical gas analyzer of the present invention, the following effects can be obtained. That is, even if the gas to be measured is a mixture of a plurality of gases and even if the light absorption spectral lines of these gases are very close to each other on the optical frequency axis, the light absorption intensities are separated by the high resolution. It is also possible to measure the gas concentration in a very small amount because of its high measurement accuracy. In this case, the noise caused by the interference of the laser light can be removed by averaging the light absorption intensities for the respective light absorption spectrum lines, so that the concentration measurement with extremely high accuracy can be realized.

In the analysis of isotope gases widely used for metabolic function tests and environmental measurements, the light absorption spectral lines of the isotope gases are very close to each other on the optical frequency axis, and Since the abundance ratio of one isotope gas is extremely small, the effect of using the optical gas analyzer according to the present invention is extremely large.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram of an optical gas analyzer according to the present invention.

FIG. 2A and FIG. 2B are characteristic diagrams of a light absorption spectrum with respect to a sample gas.

FIG. 3 is a configuration block diagram of a conventional laser spectroscopic analysis device.

[Explanation of symbols]

 2 ... Laser light source 4 ... Detector 6 ... Control circuit 8 ... Processing circuit 12 ... Sample cell 28 ... Selection circuit 30 ... Averaging circuit

Claims (1)

(57) [Claims] 1. A laser light source for generating a laser beam having a light spectrum width sufficiently narrow with respect to the light absorption spectrum line width of the measurement gas and the light absorption spectrum line spacing, and a plurality of light absorption spectra of the measurement gas. A selection circuit that selects optical frequency information of a line, a control circuit that controls the laser light source and sweeps the optical frequency of the laser light within the range of the optical frequency information, and the light intensity of the laser light that has passed through the gas to be measured. A detector for detecting, a processing circuit for extracting the light absorption intensity with respect to the optical frequency of the plurality of light absorption spectral lines of the gas to be measured from the light intensity detected by the detector, and a plurality of extracted by the processing circuit And an averaging circuit for averaging the light absorption intensity with respect to the light absorption spectrum line of the optical gas analyzer.